meson: Fix build with -Dopenjpeg=disabled

tests/check/meson.build uses the openjpeg_dep variable
unconditionally, and the subdir_done() is useless anyway, since the
plugin is only built if openjpeg_dep.found() is true. Fixes:

..\tests\check\meson.build:23:0: ERROR: Unknown variable "openjpeg_dep".

In particular, this fixes the build on UWP since we disable openjpeg
explicitly in Cerbero when building for UWP.

Part-of: <gstreamer/gstreamer!1069>
22 jobs for !1069 with gst-plugins-bad-fix-openjpeg-disabled-build in 42 minutes and 28 seconds (queued for 10 seconds)
detached
Status Name Job ID Coverage
  Trigger
passed trigger #14466232

00:00:17

 
  Build Docker
passed fedora amd64 docker #14466233

00:00:15

passed gst-indent amd64 docker #14466234

00:01:35

passed windows amd64 docker #14466235
windows shell 1809

00:00:11

 
  Preparation
passed gst indent #14466236

00:01:43

 
  Build
passed build clang fedora x86_64 #14466240

00:05:02

passed build documentation #14466244

00:08:15

manual build msys2 #14466243
docker windows 1809 allowed to fail manual
passed build nodebug fedora x86_64 #14466237

00:04:40

passed build static fedora x86_64 #14466238

00:04:27

passed build static nodebug fedora x86_64 #14466239

00:04:26

passed build vs2017 amd64 #14466241
docker windows 1809

00:09:11

passed build vs2017 x86 #14466242
docker windows 1809

00:08:45

passed cerbero trigger #14466245

00:39:47

 
  Test
passed check fedora #14466246
gstreamer

00:07:04

passed integration testsuites fedora 1/4 #14466247
gstreamer

00:09:31

passed integration testsuites fedora 2/4 #14466248
gstreamer

00:13:31

passed integration testsuites fedora 3/4 #14466249
gstreamer

00:12:01

passed integration testsuites fedora 4/4 #14466250
gstreamer

00:10:52

passed valgrind bad #14467004
gstreamer

00:08:13

passed valgrind ges #14466252
gstreamer

00:09:55

failed valgrind bad #14466251
gstreamer

00:03:42